Willy at last count U.S immigration was holding like 15,000 kids. Who had walked across the southern border without a adult. I do agree with you ship them back where they came from. I would just put them out at the gate and have them walk back. They used to fly them. But it was getting way to expensive.
Robert the Asian's have ben doing it for years. They get on a boat. Get out to the middle of the pacific and dump there papers overboard. If they are caught when the land along the west coast. They are put in a holding pens up to 30 days. Without any paperwork immigration can't prove where there from. So they are kicked loose and told to report to immigration within a few days. They don't report to anyone they just disappear into the crowed.
I have a suggestion there is a bunch of island in the south pacific that are uninhabited. Nicamarue comes to mind. It doesn't have any fresh water. But The U k , Aussie land and the U.S and the Canadians could pitch in and build a water plant and a generating station. Then dump the ileagel aliens there. And let them figure it out.